A moderate-magnitude earthquake [ML=5.6, California Institute of Technology (CIT) Seismological Laboratory] occurred at 0509 PDT on 26 April 1981 approximately 8 km north-northwest of Westmorland, Imperial County, California (figure 1). The earthquake epicenter is located by CIT Seismological Laboratory at approximately 33.102°N and 115.637°W.

The main shock of 26 April was preceded by several days of micro-to small-magnitude events. Low-level activity started on 20 April and increased on 21 April in the vicinity of the main shock (Carl Johnson, CIT Seismological Laboratory, personal communication, 1981). From the evening of 24 Apri1 unti1 the main shock, a total of 33 events of magnitude ≥3.0 occurred in this region (CIT Seismological Laboratory).

Damage from the main shock, estimated to exceed $1.5 million, was most pronounced in Westmorland, California, the closest community to epicenter. A large percentage of the buildings in Westmorland sustained some nonstructural damage from the main event. Several unreinforced adobe buildings partially collapsed and many mobile homes were jolted from their supports. City water was temporarily cut off because of damage to filtration tanks at the processing plant. In addition, the agricultural community also sustained costly losses. Much of the damage was to irrigation canals and levees, many of which partially or totally collapsed from seismic shaking.